Salt Tolerance of Tomato Plants as Affected by Stage of Plant Development

In order to simulate the usage of brackish irrigation water in greenhouse tomato (Lycopersicum esculentum Mill. cv. Daniela) culture in perlite, plants were supplied with nutrient solutions containing 0, 20, 40, and 60 mM NaCl. The three highest salinity treatments were applied at three different plant growth stages, during early vegetative growth [16 days after transplanting, (DAT)], beginning of flowering (36 DAT), and starting fruit development (66 DAT). Salt tolerance of tomato plants increased when the application of salinity was delayed. Salinity significantly decreased size and number of marketable fruits, but increased fruit quality by increasing total soluble solids and sugar content. Leaf and fruit calcium and potassium concentrations were decreased significantly by increasing salinity levels. This was compensated for the accumulation of sodium. Anion accumulation was increased by increasing chloride concentration. These results indicate that it is feasible to use brackish water for growing tomato with minimum yield losses if salt concentration and duration of exposure are carefully monitored. solution was adjusted to 5.6 with HNO3. The plants were irrigated according to the demand detected in the appropriate trays, with an excess of 25% estimated from the control plots. Three irrigation drippers were placed in each perlite bag. The treatments consisted of a control (2 dS·m) plus three salinity levels (4, 6, and 8 dS·m) in the irrigation solution. The three salinity levels were reached by adding 20, 40, and 60 mM NaCl to the basic nutrient solution (control). The three salinity levels were applied at three different growth stages, 16 d after transplanting (DAT), 36 DAT (after flowering of the first cluster) and 66 DAT (starting fruit development). Thus, there were 10 different treatments (three salinity levels × three times plus the control). The experiment was arranged as a factorial design in two randomised blocks, each containing the 10 treatments. Each treatment had three perlite bags with six plants per bag (18 plants per treatment and per block). Once a week, selective picking of ripe fruit was carried out from 29 Apr. to the end of the experiment, 6 June. The individual fruits of each truss were counted and weighed. Marketable fruits were considered as those above 50 g. Dry matter percentage after 48 h at 65 °C in a ventilated oven was determined. This material was used to determine the mineral content of fruits. Before destructive analyses of the fruit, the firmness was determined on fruit with intact skin by using a Bertuzzi FT011 penetrometer (Alfonsine, Italy), fitted with an 8-mm-diameter probe, and the surface color characters L, a and b were determined in a Minolta model CR300 colorimeter (Minolta Co., Ramsey, N.J.). Quality constituents were determined on filtrates of blended samples. Soluble solids were evaluated by an Atago N1 refractometer and expressed as °Brix at 20 °C. Acidity was determined by potentiometric titration with 0.1 M NaOH to pH 8.1, using 20 mL of juice) and reducing sugars by the anthrone method (Hewitt, 1958). The uppermost fully expanded leaves were sampled at the end of the experiment (114 DAT) and oven dried at 65° for 48 h. Chemical analyses of leaves and fruit samples were carried out after digestion with nitric and perchloric acids (2:1). Na, K, Ca, and Mg were determined by atomic absorption spectrometry (Perkin-Elmer 5500, Norwalk, Conn.). Chloride and NO3–N were extracted from 50 mg of ground material with 10 mL of deionized water. Chloride was measured by electrometric titration with a Corning chloride Analyzer 926, whereas NO3-N was determined by the difference between the absorption at 210 and 270 nm in a spectrophotometer. Vegetative and fruit yield components, fruit quality constituents and leaf and fruit mineral composition data were subjected to analysis of variance (ANOVA) using the SPSS computer package (SPSS, 1985). Salt tolerance was determined by adjusting the data with nonlinear regression using the sigmoidal model of Van Genuchten (1983), where Yr = Ym / [1 + (C/C50)], Yr is the Valuable agriculture/food production in arid and semiarid regions of the world, which depends on irrigation, faces a serious challenge because it must increase or at least maintain crop productivity while coping with ever more saline irrigation waters. The success of using such waters needs advances in the knowledge of the many factors involved in plant salt tolerance (Maas and Hoffman, 1977). The markets demand horticultural crops of high quality throughout the year, which encourages farmers to grow crops in artificial substrates and in greenhouses. Under proper management, soilless culture will permit use of low quality waters, since it is more feasible to control salt concentration in the root media by establishing the appropriate leaching fraction. Worldwide, tomato is one of the most important horticultural crops. The effect of salinity on fruit yield and quality has been investigated using different substrates, such as soils (Martinez et al., 1987; Mitchell et al., 1991a; Shalhevet and Yaron, 1973), solution cultures (Cerdá et al., 1977), coarse sand (Mizrahi et al., 1988), peat-loam (Sharaf and Hobson, 1986), nutrient film system (Adams and Ho, 1989; Gough and Hobson, 1990) and rockwool (Adams, 1991), but there are no data on perlite. Crop salt tolerance is influenced by several factors. The growth stages at which salinization is initiated, the final level of salinity achieved, and the portion of the plant to be marketed (Lunin et al., 1963). Cultivar differences must also be considered in evaluating crop salt tolerance, since many crops are developed from a diverse genetic base (Maas and Hoffman, 1977). In soilless culture, the physical properties of the substrate may influence salt tolerance, and consequently crop production and fruit quality (Kuehny and Morales, 1998). This paper describes a water management strategy, by exploring the response of tomato to saline water applied at different development stage, and by establishing the maximum salt concentration in the irrigation water to maintain crop productivity. The objective of our study was to determine the effect of different degrees of salinity in the irrigation solution, applied at different growth stages, on vegetative growth, fruit yield and quality, and fruit and leaf mineral composition. Materials and Methods Tomato (Lycopersicum esculentum Mill. cv. Daniela) plants were grown in perlite bags, 1.2 m long and 20 cm high, in a greenhouse equipped with a computer regulated system for drip irrigation, under controlled environmental conditions. During the experiment, the daytime temperature was maintained between 20 to 30 °C and the nighttime temperature was never lower than 15 °C. The relative humidity (RH) was 55% throughout the day, whereas at night it reached 75%. The plants were transplanted on 19 Jan. 1998, and irrigated with a basic nutrient solution of the following macronutrient composition (mM): NO3, 14; H2PO4, 1.5; SO4, 1.5; Ca, 4; K, 7.5; and Mg, 1.5. Micronutrient concentrations were (in μM): Fe, 17.9; Mn, 9.1; Zn, 7.6; B, 23.3; Cu, 0.31; and Mo, 0.1. The pH of the 6595, p. 1260-1263 12/19/01, 1:04 PM 126